tonihei e250fe6277 Make sure not to create new playback sessions while still IDLE.
The first session should only be created once we have the media items
and/or called prepare. Otherwise the first session is created with
an EventTime having an empty timeline making it less useful.

issue:#7193
PiperOrigin-RevId: 308100555
2020-04-27 10:41:33 +01:00
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